As the sizes of DRAM elements are shrinking down, aspect ratio of peripheral contact of the DRAM elements will increase as the contact size reduces.
DRAM peripheral contact is usually formed after capacitor top plate (poly) patterning and capacitor planarization. A one-step contact etching is difficult considering the decrease in photoresist thickness due to the employment of Deep Ultra Violet (DUV). High aspect ratio contact generally results in difficulty in contact etching such as contact profile and also barrier metal deposition. A non-90 degree profile will result in extremely small size of bottom contact and extremely high contact resistance.
To overcome these drawbacks, stacked contact is a way to reduce contact aspect ratio. However, misalignment (13) between tungsten plugs (10 and 11) may result in volcano effect as shown in FIG. 1.